The invention relates to a zero voltage switch, comprising two parallel connected current branches each of which takes over a direct current provided by a current source in one half wave of the alternating voltage, this direct current being coupled out for the purpose of producing control pulses via a third current branch in the zero crossing phases of the alternating voltage, during which phases both current branches are blocked. The invention is particularly suitable for triggering a triac. Such a circuit is known for example from German Offenlegungsschrift No. 25 53 764.
Zero voltage switches are used, by way of example, to trigger full-wave thyristors (triacs). The triac is to be triggered during the zero axis crossing of an alternating voltage signal in order to reduce to a minimum as far as possible the disturbances in the alternating current mains which are caused by switching. Therefore, in the case of the known zero axis crossing detector, the pulses which ensure that the triac is conductive are produced during the zero axis crossing phase of the alternating voltage and the pulse width is distributed uniformly between the positive and negative half waves of the alternating voltage signal. This uniformity is achieved by providing the zero axis crossing detector with two current paths which are connected in parallel and are provided with bi-polar transistors. These current paths each taking over a direct current supplied by a constant current source in one half wave in each case of the alternating voltage, the direct current being coupled out via a third transistor only in the zero axis crossing phase and serving to produce the output pulses.
The known circuit is equipped with multi-collector current image transistors which ensure that the output pulses are exactly symmetrical at the zero axis crossing of the alternating voltage. In the known circuit, the alternating voltage is still so large, when the first control pulse is triggered after a switching command has been received, that high frequency disturbance is triggered by phase gating in the half wave which has just passed and this disturbance may occur for example in the form of clicking or crackling noises in associated radio equipment.